1. Field of the Invention
The present invention relates to a readily sinterable silicon nitride powder for use as a starting material for sintered silicon nitride ceramics useful as structural ceramics, and particularly, for silicon nitride ceramics having excellent strength at high temperatures.
2. Description of Related Art
Recently, because of their superior characteristics concerning strength, toughness, and corrosion resistance, more silicon nitride ceramics are used in various fields as structural materials or machine parts at a temperature not higher than 1,000.degree. C. Generally, an addition of from about 5 to 10% by weight of an oxide such as Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 is indispensable in sintering silicon nitride. However, these additives have lead to the problematic formation of a grain boundary phase which readily undergoes softening at a high temperature. To prevent the drop in strength at high temperatures due to the formation of a grain boundary phase, etc., and to obtain a sintered silicon nitride ceramics capable of maintaining high strength in a temperature range of from room temperature to 1,500.degree. C., not only a sintering aid such as Yb.sub.2 O.sub.3 is proposed, but also a powder having favorable properties for use as a starting material of a sintering is developed.
Silicon nitride powder has been prepared heretofore by means of, for example, (1) direct nitridation of metallic silicon powder; (2) carbothermal reduction and nitridation of silica powder; and (3) imide decomposition process which comprises reacting a silicon halide with ammonia. The silicon nitride powders thus prepared by different methods enumerated above yield approximately the same impurity concentration, oxygen concentration, particle diameter, etc., however, their sinterability and the properties of sintered bodies that result therefrom greatly differ from each other depending, assumably, on the processing history. In general, the silicon nitride powder prepared by the imide decomposition process (3) described hereinbefore is believed to be the most easily sinterable one, and to yield sintered bodies having superior characteristics.
With progress in the research on the relation between the powder characteristics and the sinterability as well as the properties of the sintered bodies resulting from the powder, and with the factors dominating the sinterability of a powder and the properties of the sintered bodies resulting therefrom becoming better understood, it has been found that the production history is not the absolute factor which poses influence on the sinterability of a powder and the properties of the sintered bodies resulting therefrom, and that various powder characteristics mutually react with each other to have influence on the sinterability, etc., of a powder. This is described in further detail below.
Silicon nitride is available in two different crystalline phases; i.e., .alpha.-and .beta.-phase. The .beta.-phase is an oxygen-free pure crystalline silicon nitride, but the .alpha.-phase takes up oxygen into the crystal lattice thereof to form a solid solution with oxygen. During the heating step in sintering silicon nitride, silica on the surface of the silicon nitride particles reacts with the sintering aid as to generate a liquid phase. Hence, during the sintering process, silicon nitride dissolve into the liquid phase and reprecipitates in the form of a .beta.-phase as to provide a dense sintered body. Accordingly, it is believed that a silicon nitride powder containing 85% or more of an .alpha.-phase is preferred for use as a starting material for producing a sintered body.
However, no convincing conclusion is derived to the present on the correlation between the phase composition (.beta.-phase fraction, .alpha.-phase fraction, and amorphous phase fraction) of a silicon nitride powder containing .alpha.-phase at a concentration of 85% or higher and the sinterability or the properties of the sintered bodies which results from the powder. Furthermore, the mutual influence that is cast to each other between the phase composition and the other powder characteristics is yet to be clarified. Thus, it has been found very difficult to stably produce a sintered silicon nitride ceramics having high strength at high temperature, and still, at a high reproducibility.